This application is a continuation-in-part of U.S. application Ser. Nos. 260,813 filed Oct. 21, 1988, and 371,703 filed June 26, 1989 (both now abandoned), and U.S. application Ser. No. 444,727 filed Dec. 1, 1989, now U.S. Pat. No. 4,971,731.
This invention relates to the separation of minerals in finely comminuted form from an aqueous pulp by froth flotation process, and especially to a froth flotation system with an improved means for introducing the gaseous medium in the form of minute bubbles into the liquid flotation column. More particularly, the invention relates to a device for generating gas bubbles in a flowing stream of aqueous liquid and delivering the bubble containing stream to the flotation column.
Commercially valuable minerals, for example, metal sulfides, apitictic phosphates, and the like, are commonly found in nature mixed with relatively large quantities of gangue materials. As a consequence, it is usually necessary to beneficiate the ores in order to concentrate the mineral content. Mixtures of finely divided mineral particles and finely divided gangue particles can be separated and a mineral concentrate obtained therefrom by widely used froth flotation techniques.
Froth flotation involves conditioning an aqueous slurry or pulp of the mixture of mineral and gangue particles with one or more flotation reagents which will promote flotation of either the mineral or the gangue constituents of the pulp when the pulp is aerated. The conditioned pulp is aerated by introducing into the pulp minute gas bubbles which tend to become attached either to the mineral particles or the gangue particles of the pulp, thereby causing one category of these particles, a float fraction, to rise to the surface and form a froth which overflows or is withdrawn from the flotation apparatus. The other category of particles, a non-float fraction, tends to gravitate downwardly through the aqueous pulp and may be withdrawn at an underflow outlet from the flotation vessel. Examples of flotation apparatus of this type are disclosed in U.S. Pat. Nos. 2,753,045; 2,758,714; 3,298,519; 3,371,779; 4,287,054; 4,394,258; 4,431,531; 4,617,113; 4,639,313; and 4,735,709.
In a typical operation, the conditioned pulp is introduced into a vessel to form a column of aqueous pulp, and aerated water is introduced into the lower portion of the column. An overflow fraction containing floated particles of the pulp is withdrawn from the top of the body of aqueous pulp and an underflow or non-float fraction containing non-floated particles of the pulp is withdrawn from the column in the lower portion.
In several systems of this type, the aerated water is produced by first introducing a froth or surfactant into the water and passing the mixture through an inductor wherein air is aspirated into the resulting liquid. In order to obtain the required level of aeration, a high flow rate for the water must be maintained through the inductor. While recirculation systems have been devised to minimize the amount of "new" water added to the system, a significant expenditure in energy is required to move such large quantities of water.
Another problem encountered results from the difference between the concentrations of solid particles contained in slurries of different minerals. Phosphates, for example, do not typically require extensive grinding in order to liberate the desired mineral components of the pulp. As a result, the aqueous slurry or pulp fed to the flotation apparatus typically consists of approximately seventy-five percent (75%) solids and twenty-five percent (25%) water. Sulfides, on the other hand, approach the opposite extreme, and typically require extensive beneficiation through grinding of the material to a very fine state in order to liberate the desired minerals from the gangue.
The addition of water throughout the sorting, grinding, and classifying stages of the beneficiation process results in an aqueous slurry comprising approximately ten percent (10%) solid matter and ninety percent (90%) water. Thus, the addition of significant additional amounts of water is undesirable in that significant amounts of the finely ground valuable minerals may avoid capture by the aeration bubbles and remain suspended in the liquid component of the slurry.
Another method for introducing minute air bubbles into the flotation vessel comprises a sparging system such as that disclosed in U.S. Pat. No. 4,735,709. Spargers or microdiffusers are normally tubular members formed of porous material such as sintered stainless steel, porous plastic, ceramic or the like, with a porous wall having a typical average pore size of about 50 microns. The sparger is placed within the flotation vessel and air under pressure is introduced into its interior. The pressurized gas or air within the interior chamber is forced through the pores and into the aqueous pulp in the flotation chamber.
While spargers are used with considerable success, they do have certain disadvantages, including the tendency of the small pores to become clogged with contaminants.
The method and apparatus of the present invention, however, resolve the differences indicated above and afford other features and advantages heretofore not obtainable.